1. Field of the Invention
The present invention relates to an magneto-optical recording medium, and in particular, to a magneto-optical recording medium having a protective film with an increased Kerr effect enhancement and an improved protection characteristic.
2. Description of the Prior Art
For magneto-optical recording media, there have been powerfully conducted researches and developments to implement high-density recording media having a magnetic film and being capable of rewriting data recorded thereon.
Among magneto-optical recording substances employed to constitute s magnetic film for such a recording medium, an amorphous alloy comprising a rare-earth metal and a transition metal (to be simply refferred to as an RE-TM alloy herebelow) has such features that there is attained a vertical magnetization film, namely, a direction of magnetization thereof is vertical to a film growth plane, that there is developed a large coersive force of several units of KOe, and that the film can be manufactured in a relatively easy fashion by use of a sputtering technology, a vaporized evaporation, or other deposition technology. In this regard, consequently, the research and development has been greatly advanced for the RE-TM alloy, which hence has been put broadly to practical uses.
However, a magnetic film formed with an RE-TM alloy is weak against corrosive actions (reference is to be made to description in page 427 of "Hikarijiki Disk" supervised by Nobutake Imamura and published from Triceps Co., Ltd.); moreover, the magnetic film is attended with a disadvantage that there is not attained a satifactorily great the magneto-optic Kerr effect.
To overcome this difficulty, there has been known a film structure in which a magnetic film formed with an RE-TM alloy is placed between protective films including various materials so as to prevent corrosions of the magnetic film and to increase the virtual rotation angle associated with the Kerr effect by use of a multi-reflection of a light (reference is made also to description in page 119 of the article above written by Imamura et al.)
As can be understood from the description of the prior art technology above, in the magneto-optical recording medium, a refractive index and a transmittivity of light through the a protective film exert a great influence on characteristics associated with information write and read operations. As for the refracive index, for example, a protective material mainly including aluminum such as AlN, AlSiN, or AlSiON has a refractive index n of about two and hence has been known as a material developing a relatively high index value among the substances heretofore reported. At present, however, with development of various information apparatuses, there has been required an improvement in the recording density, which leads to a problem that the refractive index developed by such a protective film material of the prior art technology cannot attain a satisfactory Kerr effect enhancement.
Furthermore, as described above, for the protective film formed on a read side of the magnetic film, it is required that there is developed a small extinction coefficient k with respect to wavelength adopted in the information write and read operations. The extinction coefficient k is attained as an complex refractive index n-ki (i is an imaginary unit) including the refractive index n. In actual, the extinction coefficient k takes a value of about 10.sup.-1 in the known protective film materials; however, it is desirable to develop a further reduced value.
As a problem in this situation, there has not been produced a protective film material which satisfies the requirements related to the refractive index n and the extinction coefficient k and which is satisfactorily resistive against corrosions.